Every conductive object possesses a characteristic capacitive coupling to every other conductive object and the ground plane of the earth, the magnitude of that coupling being a function of the object's size and its proximity to other conductive objects. The capacitive coupling between two metal antennas, or between a single antenna and the ground plane, will be increased by the nearby presence of a human body, which by virtue of the salts dissolved in the intercellular and extracellular fluids will behave as a conductor.
This elementary fact has been known for many decades, and has served as the basis for the design of a great variety of electronic devices which can detect the approach of a person to a metal antenna by sensing the increase in the capacitance of the antenna that the person produces. These devices are generically known as capacitance proximity sensors, and can be divided into classes on the basis of the method employed to measure the capacitance of the antenna.
Most of the existing devices employ one of two basic methods, which I will refer to as the resonance conversion method and the reactance conversion method.
In the resonance conversion method the capacitance of the antenna is converted into a frequency by utilizing the antenna as the capacitive element in an inductance-capacitance oscillator. The frequency of oscillation will be proportional to the inverse square root of the antenna capacitance, and a change in capacitance caused by the approach of a person will be reflected in a change in frequency. Any of many standard methods can be employed to monitor the oscillator frequency and provide an indication when the frequency deviates beyond some preset amount.
In the reactance conversion method the capacitance of the antenna is converted into a voltage. The antenna is driven by a high impedance signal source, and the signal at the antenna is rectified and filtered. The capacitive reactance of the antenna will load the signal source, and the signal amplitude will be inversely proportional to the antenna capacitance, the signal frequency, and the signal source impedance. An increase in antenna capacitance caused by the approach of a person will thus be reflected in a reduction in signal level, and hence in the output voltage of the rectifier.
While both these methods work very well on a laboratory bench, their practical application has been limited by a necessary compromise between sensitivity and noise resistance. Transients with an amplitude of several hundred to several thousand volts are routinely found on residential power lines, and these transients are fed into the proximity sensor by capacitive coupling between the sensor antenna and the power lines. Transients of sufficient amplitude can cause momentary alterations in both the frequency of a resonance convertor and the apparent reactance of the antenna, so in order to prevent an erroneous response to electrical noise a limit must be placed on the sensitivity of the proximity sensor.
With most sensor designs this limit is so low that the sensor is useless for practical purposes, and while some successful attempts have been made to reduce susceptibility to noise without sacrificing sensitivity, they have so far led to designs too complex--and therefore expensive--to be economically practical.